Maintaining constant temperatures and atmospheres in furnaces, boilers, and other combustion chambers using gaseous hydrocarbons as the source of heat requires the employment of a fuel with a fairly constant composition and heating value and an oxidant (usually air) in definite proportions to the fuel. Worldwide, the use of producer gas, town gas, and other low, variable heating value gases has been superseded by the use of natural gas which has had an acceptably uniform heating value for most uses. However, present changes in natural gas distribution toward large pipeline networks and regasification of liquified natural gas may result in gas with some variation in composition. For most thermal processes the magnitude of the variation may be acceptable; however, in others it may not.
Furthermore, the trend toward coal gasification will once again introduce wider variations and low heating value gas into industrial processes. A number of solutions exist for utilizing these variable heating value gases in thermal processes requiring constant heat. For example, one method involves constantly measuring the changes in temperature produced by the burning fuel of variable calorific value and then varying the mass flow of gaseous fuel to compensate for these changes. This is shown by U.S. Pat. No. 2,780,414 to De Heer wherein the temperature is also observed in an auxiliary burner. Other U.S. Pat. Nos. to Kappel (2,818,246), Andrews (3,407,022), Dailey (2,866,602), and Schmidt (1,849,335; 1,933,641; 2,349,521) disclose various ways of controlling air to fuel ratios in burning zones. However, they are directed mainly to controlling the amount of heat produced and not to controlling stoichiometry of the gas and oxidant as is the present invention.
In ordinary burners wherein the ratio of air to fuel gas is mechanically fixed or wherein the air is admitted by inspiration, the changing of the mass flow of fuel gas to compensate for calorific variations can result in changed burning conditions and atmospheres inside the combustion zones. For example, rich fuel/air mixtures can result in pollutants in the form of unburned hydrocarbons and carbon monoxide and in a reducing atmosphere in the combustion zone. Lean mixtures can result in an oxidizing atmosphere in the combustion zone, excessive nitrogen oxides and in heat losses as a result of having to heat excess air which is then lost in the exhaust.
One present method of controlling the fuel to air ratio consists of measuring the gas composition (particularly the oxygen level) of the exhaust gas with an analyzer and feeding the information back to the feed gas and air lines to compensate for variations. In some processes, however, this remedial type of compensation can be slow, and damage to the products being thermally treated may occur before the change is made. In addition, some analyzers work only on the one side of the stoichiometric ratio.